Biomedical Engineering Reference
In-Depth Information
to temperature changes and induces vasodilation or re-
striction as necessary to control and redistribute blood
volume regulating core and peripheral body temperatures.
Most volatile agents impair vasoconstriction, potentially
altering blood-volume distribution. Greater blood volume
in extremities increases heat loss, thus lowering core body
temperature. Muscle relaxants and anesthesia otherwise
clearly inhibit shivering and reduce heat production by
resting muscles (Morley-Forster, 1986).
Accidental hyperthermia during anesthesia is not
common in the United States, as most all ORs are air-
conditioned. Hyperthermia is more common in tropical
climates, in ORs without air conditioning. Malignant hy-
perthermia can be detected by monitoring the patient's
temperature, but an increase in expired carbon dioxide is
an earlier indicator.
Thermistors are most commonly used to measure the
patient's temperature. Their use poses little risk to the
patient; they are reliable and relatively simple to use,
and they provide an accurate measurement when used
correctly. The most frequent measurement location is
nasopharyngeal. The most common problems are cooling
from respiratory gases from leaking and misplacement.
These devices generally consist of probes that connect to
a temperature-sensing device, either a stand-alone box or
integrated into a cardiovascular monitor. Thermistors are
also used in pulmonary artery catheters, which are used
for invasive pressure monitoring and provide a core-
temperature measurement.
Infrared tympanic membrane measurement can pro-
vide an accurate core temperature but is susceptible to
user error. An infrared scanning device with disposable
protective cap is inserted in the ear canal, where the
energy radiated from the tympanic membrane is mea-
sured, converted, and displayed as a temperature.
Liquid crystal thermometers are available to measure
skin temperature. However, because skin temperature
does not correlate well with core temperature, their use
is not practical in anesthesia.
ventilation. The drugs used are characterized as depola-
rizing or nondepolarizing. They work by blocking neu-
romuscular transmission, thereby reducing the muscle's
ability to contract, and paralyzing the patient. Paralysis
can be achieved also at high doses of volatile anesthetic
agents. One important distinction is that a patient can
be paralyzed but may not necessarily be anesthetized.
Failure to render the patient unconscious results in
a most undesirable situation in which the patient can
experience excruciating pain of surgery yet, because of
paralysis, is unable to cry out or otherwise signal to the
surgeon.
Peripheral nerve stimulators (also known as ''twitch
monitors'') are relatively simple pulse generators. Most
stimulators are battery-powered devices that connect to
the patient by means of a cable and electrodes. Most
often, the electrodes are similar to those used for ECG
monitoring but can be specific for the application.
Most monitors provide information to the physician by
pure observation. The user must feel and observe the
patient's reaction to the electrical stimuli. There are
units available that can monitor the patient's reaction
electrically and/or mechanically and can display patient
characteristics (electronic or paper chart recorders).
These units are used less regularly, often for teaching
purposes.
Most frequently, the electrodes are placed over the
ulnar nerve near the wrist. The location preference is
primarily related to muscle innervation. The ulnar nerve
solely innervates the adductor pollicis muscle. Stimu-
lating the ulnar nerve minimizes the possibility of
crosstalk with other muscles, allowing the user to mon-
itor for a response only on the patient's thumb. Less
frequently, the electrodes can be placed on a facial nerve
if access to extremities is difficult.
Twitch monitors generate a single-phase DC pulse,
normally of a fixed duration and adjustable current.
Preprogrammed units are available that deliver pulses in
sets to help the physician to identify the state of neuro-
muscular blockade. The most common pulse sets are:
Single twitch
d
upon demand, every 10 seconds or
Peripheral nerve stimulators
1 per second
Train of four
a set of four pulses at half-second
intervals or can be set to repeat every 12 seconds
Double burst
d
Peripheral nerve stimulators are used to provide an in-
dicator of neuromuscular blockade or otherwise general
paralysis. During anesthesia, muscle relaxants are most
frequently used to relieve natural muscle tension to fa-
cilitate intubation and surgery. In an ICU, muscle re-
laxants can be used to aid the use of mechanical
a set of three pulses, 20 milliseconds
apart, followed by another similar burst 750
milliseconds later
Tetanus
d
d
repetitive, single pulses at 50 Hz or greater
